【分布式锁】02-使用Redisson实现公平锁原理

前言

前面分析了Redisson可重入锁的原理，主要是通过lua脚本加锁及设置过期时间来保证锁执行的原子性，然后每个线程获取锁会将获取锁的次数+1，释放锁会将当前锁次数-1，如果为0则表示释放锁成功。

可重入原理和JDK中的可重入锁都是一致的。

Redisson公平锁原理

JDK中也有公平锁和非公平锁，所谓公平锁，就是保证客户端获取锁的顺序，跟他们请求获取锁的顺序，是一样的。公平锁需要排队，谁先申请获取这把锁，谁就可以先获取到这把锁，是按照请求的先后顺序来的。

Redisson实现公平锁源码分析

公平锁使用也很简单：

1RLock lock = redisson.getFairLock("anyLock");
2lock.lock();
3lock.unlock();

核心lua脚本代码：

 RFuture tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand command) {
    internalLockLeaseTime = unit.toMillis(leaseTime);

    long currentTime = System.currentTimeMillis();    
    if (command == RedisCommands.EVAL_LONG) {
        return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, command,
                // remove stale threads
                "while true do "
                + "local firstThreadId2 = redis.call('lindex', KEYS[2], 0);"
                + "if firstThreadId2 == false then "
                    + "break;"
                + "end; "
                + "local timeout = tonumber(redis.call('zscore', KEYS[3], firstThreadId2));"
                + "if timeout <= tonumber(ARGV[4]) then "
                    + "redis.call('zrem', KEYS[3], firstThreadId2); "
                    + "redis.call('lpop', KEYS[2]); "
                + "else "
                    + "break;"
                + "end; "
              + "end;"
                
                  + "if (redis.call('exists', KEYS[1]) == 0) and ((redis.call('exists', KEYS[2]) == 0) "
                        + "or (redis.call('lindex', KEYS[2], 0) == ARGV[2])) then " +
                        "redis.call('lpop', KEYS[2]); " +
                        "redis.call('zrem', KEYS[3], ARGV[2]); " +
                        "redis.call('hset', KEYS[1], ARGV[2], 1); " +
                        "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                        "return nil; " +
                    "end; " +
                    "if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " +
                        "redis.call('hincrby', KEYS[1], ARGV[2], 1); " +
                        "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                        "return nil; " +
                    "end; " +
                        
                    "local firstThreadId = redis.call('lindex', KEYS[2], 0); " +
                    "local ttl; " + 
                    "if firstThreadId ~= false and firstThreadId ~= ARGV[2] then " + 
                        "ttl = tonumber(redis.call('zscore', KEYS[3], firstThreadId)) - tonumber(ARGV[4]);" + 
                    "else "
                      + "ttl = redis.call('pttl', KEYS[1]);" + 
                    "end; " + 
                        
                    "local timeout = ttl + tonumber(ARGV[3]);" + 
                    "if redis.call('zadd', KEYS[3], timeout, ARGV[2]) == 1 then " +
                        "redis.call('rpush', KEYS[2], ARGV[2]);" +
                    "end; " +
                    "return ttl;", 
                    Arrays.